The present application relates to the decoupling of radio frequency coils. It finds particular application to the decoupling of receive coils used in magnetic resonance (MR) applications.
MR imaging has proven to be a valuable technique for providing information about the internal structure and function of an object under examination. In medical imaging, for example, MR imaging techniques are widely used to provide information on the physiology of human patients.
One limitation, however, on the utility of images and other information generated by MR scanners is the effect of electronic noise. Indeed, signal to noise ratio (SNR) is a key parameter used to evaluate the quality of the information generated by an MR system.
Various techniques have been used to improve MR system SNR. As increasing the strength of the main magnetic field increases the strength of the resultant MR signals, there has been an ongoing trend toward the use of scanners having ever higher field strengths. However, systems incorporating higher field strength magnets are generally more complex, larger, and more expensive than lower field strength systems. Physiological and other limitations, such as Specific Absorption Ratio (SAR) limits, also complicate the move to higher field strengths. Moreover, even for a system operating at a given field strength, it remains desirable to provide a relatively higher SNR.
Another technique for compensating for SNR performance and improving image quality has been the use of relatively longer scanning times. However, increasing the scanning time can lead to increased motion artifacts, have a deleterious impact on patient comfort, and results in the less efficient utilization of the scanner. Again, for a given imaging time, it also remains desirable to provide a relatively higher SNR.
Yet another technique has been the use of anatomy specific radio frequency (RF) coils which provide an improved SNR than the large body receive coil built into the MR scanner. One technique has seen the adoption of surface and other coils having a relatively small field of view (FOV). While reducing coil FOV tends to reduce the effects of sample generated noise (e.g., body or tissue noise in a human patient), the smaller FOV can be problematic when imaging relatively larger areas such as the brain, spine, or heart.
As a result, coils having multiple smaller coils have been developed. A particular advantage of such array coils is their ability to provide a relatively larger FOV while still providing higher sensitivity and lower noise performance. Unfortunately, however, inductive coupling between the individual coils in the array can have a deleterious effect on the performance of the overall coil array and the resultant image quality.
Various techniques have been used to address this receive-receive coupling issue. In one technique, the various coils in the array of coils are partially overlapped. While such a technique can be effective for reducing the mutual inductance between the coils, physical constraints can make it difficult to implement, especially when the array contains many small coils. However, to maintain the FOV, this approach requires that each element be slightly larger, typically in the range of 25% to 40%, so that the coils can be overlapped. Increasing the size of the individual loops effectively reduces the SNR benefit of using arrays of smaller coils. It can also be difficult to implement when the coils are implemented on a planar or otherwise non-conformable substrate, as may occur when, for example, the coils are fabricated from high temperature superconductors (HTS) or other superconducting materials which typically use a substrate such as sapphire or lanthanum aluminate.
In another technique associated with parallel imaging such as SENSE or SMASH, the array coils are designed such that there are gaps between the individual elements. In this design, decoupling the coils via overlapping is not possible and a capacitive or inductive technique is used to decouple the coils. However, the capacitors and inductors introduce impedance into the circuit. This impedance degrades noise performance, and can be especially significant as the number of coils and decoupling circuits in the array is increased.
Capacitive and low input impedance pre-amplifier decoupling techniques have also been implemented. Unfortunately, however, these techniques can degrade the SNR of the coil, especially where the coil includes a relatively large number of coil elements or where body noise is relatively small. Moreover, these techniques are relatively difficult to implement in an HTS coil array without degrading the quality factor of the HTS coil.